Cardiac output or blood flow is one of the key indicators of the performance of the heart. Blood flow can be defined as volume of fluid flow per time interval. Fluid velocity is a function of flow area at the measurement site. Use of blood flow measurements allows discrimination between physiologic rhythms, such as sinus tachycardia, which is caused by exercise or an emotional response, and other pathologic rhythms, such as ventricular tachycardia or ventricular fibrillation.
Cardiac arrythmia is defined as a variation of the rhythm of the heart from normal. The cardiac heartbeat normally is initiated at the S-A node by a spontaneous depolarization of cells located there during diastole. Disorders of impulse generation include premature contractions originating in abnormal or ectopic foci in the atria or ventricles, paroxysmal supraventricular tachycardia, atrial flutter, atrial fibrillation, ventricular tachycardia and ventricular fibrillation. Ventricular arrhythmia can occur during cardiac surgery or result from myocardial infarction. Ventricular tachycardia presents a particularly serious problem because the patient, if left untreated, may progress into ventricular fibrillation.
Blood flow measurements allow discrimination between normal and pathologic rhythms by providing a correlation between the electrical activity of the heart and the mechanical pumping performance or fluid flow activity of the heart. During sinus tachycardia, an increase in heart rate will usually be accompanied by an increase in cardiac output or blood flow. During ventricular tachycardia or ventricular fibrillation, heart rate increase will be accompanied by a decrease in, or perhaps a complete absence of, cardiac output or blood flow. A number of important cardiac and clinical devices may be improved by a more accurate measure of cardiac output. The ability to measure blood flow can be applied to the following four areas: (1) automatic implantable defibrillators, (2) rate adaptive pacemakers, (3) cardiac output diagnostic instruments and (4) peripheral blood flow instruments.
Prior art methods of measuring blood flow have included blood thermal dilution, vascular flow monitoring, and injectionless thermal cardiac output. U.S. Pat. No. 4,785,823 to Eggers, et al., which is incorporated herein by reference, teaches a thermal dilution catheter utilizing a pair of spaced electrodes in electrical contact with blood flowing adjacent to a catheter. In accordance with that patent, a potential difference is applied across the electrode which causes electric current to flow in the volume of blood in the region of the electrode, thereby creating a bolus of blood at elevated temperature suitable for measurement of blood flow by the thermal dilution principle. The device heats the blood by passing an electrical current between two or more electrodes, each of which is an electrical contact with the blood. The electrodes are mounted on a catheter or other similarly longitudinal support member which is inserted longitudinally in the blood vessel in which blood flow is to be measured. An AC electrical potential difference is applied to the electrodes. A temperature sensor is located in the blood stream downstream from the electrodes by being mounted on the support member. The temperature sensor produces an output indicative of the temperature of the blood adjacent to the sensor. This output is proportional to a conventional thermal dilution curve and provides an accurate measurement of the rate of blood flow between two spaced locations. The Eggers, et al. invention heavily depends on the fact that blood has a lower electrical resistance and, thus, the current will not take a path through proximally adjacent tissue. The device is intended for use as an insertable device that is fed into the right atrium, right ventricle and into the pulmonary artery. A device incorporating this invention was constructed but used an energy pulse of 100 volts RMS at 1 amp RMS at a frequency of 200 to 500 KHz for a duration of 2.5 seconds.
In U.S. Pat. No. 4,419,999 to James W. May, Jr., et al., incorporated herein by reference, there is described a method and apparatus for monitoring vascular flow wherein the device measures blood flow through a blood vessel utilizing the principle of energy conversion to heat by myocardial activity, organ metabolism, and laminar frictional flow in blood vessels. The device measures heat dissipation through the vessel wall with an obstruction to flow. The device is placed next to the vessel wall and an output signal correlates blood flow with temperature. The device is used in repaired vessels to study blood flow. The device's leads are brought through the skin and attached to a temperature monitor. It works by measuring the temperature of blood vessels both proximal and distal to an anastomotic repair. This is done by measuring temperatures that exist ambiently within the body on a continual basis by providing heat to the system and noting the rates of dissipation proximal and distal to the anastomotic repair. The device includes a thermal sensor which may be placed over a portion of the blood vessel with leads taken out transcutaneously to a temperature monitor.
Another U.S. Pat. No. 4,819,655 to William E. Webler, describes a method and system using a catheter which is configured such that fluid can circulate around it when the catheter is in position. The fluid may circulate down one lumen as far as the right atrium or ventricle over to another lumen and back up and out of the catheter. In using that invention, a solution is not injected into the blood stream. The catheter temperature sensor is positioned to monitor mixed venous blood temperature. A temperature sensing device monitors a fluid temperature at the inlet and outlet from the catheter. After steady state fluid circulation is initiated, a circulatory fluid cools the blood through the lumen walls. It also cools any adjacent fluid to the lumen which also cools the blood through the lumen wall and through dilution of heat introduced by the IV solution. An appropriate heat balance equation allows the cardiac output to be calculated. The method has a duty cycle and utilizes an equilibrium/disequilibrium measurement cycle.
In U.S. Pat. No. 4,865,036 to Chirife, entitled "Antitachyarrhythmia Pacemaker Using Pre-Ejection To Distinguish Physiologic From Pathologic Tachycardia," a cardiac apparatus for cardioversion or defibrillation is provided while, in the event of pathologic tachycardia or ventricular fibrillation, the rate of heart depolarization is compared to a predetermined heart rate value indicative of the onset of tachycardia. The heart's pre-ejection period is monitored to determine whether an increase in heart rate above the predetermined value is accompanied by a decrease in the pre-ejection period. If not, a pathologic rather than physiologic episode is diagnosed and a cardioversion pulse protocol is initiated. By also monitoring the mechanical pulse of the heart, ventricular fibrillation is diagnosed and the cardioverters prepare to shock the heart back into sinus rhythm. The mechanical pulse rate tracks the electrical rate of depolarization and tachyrhythmia is confirmed. The diagnosis may be further confirmed by taking into account the rate at which the heart rate increases.
The Sekii, et al. U.S. Pat. No. 4,979,514 entitled "Cardiac Output Measurement Method And System For The Application Of Same" relates to blood flow measurements, but differs from the present invention in that it incorporates a blood flow velocity measurement device with a cardiac output device and, as such, it is not applicable to an implantable arrangement. In addition, the method described in the Sekii, et al. patent for measuring blood velocity is different from the method outlined in that path.
The present invention differs substantially from the prior art in many respects. Initially, the invention is implantable, long-term, in a patient by way of a catheter. Further, continuous measurements of intravenous blood flow are possible with the invention. The present invention utilizes an extremely low-power solution. Moreover, the invention is unobtrusive and does not require the use of auxiliary terminals, fluids, catheters or lumens to measure the performance of the heart.
The novel blood flow sensor of the present invention is based on heat transfer principles. The heat induced into a fluid stream is carried away by the fluid via convection. The heat flow from the point of heat introduction to a downstream sensing position can be detected as a temperature difference between the two positions. Heat flow is then inversely proportional to the temperature difference between the two points. Heat flow can then be converted directly to fluid flow by knowing the thermal properties of the fluid and the dimensions in which the flow takes place. Prior art radially activated thermocouple elements have been designed for measurements of flow within a pipe. A good example of a radially activated thermocouple can be found in U.S. Pat. No. 4,460,802 to Paul Beckman which is incorporated herein by reference.
Present generation automatic implantable cardiac defibrillators or AICD devices have difficulty discriminating between the physiologic rhythms and pathologic rhythms of the heart. This limitation is due to the fact that electrical activity of the heart is used for detection of ventricular tachycardia or ventricular fibrillation rather than a mechanical indicator such as blood flow. Improper discrimination between the electrical and mechanical activity of the heart causes an incorrect interpretation of physiologic need.
Prior art rate-responsive pacemakers use various types of sensors to detect the exercise level or emotional response of the patient. Sensors presently employed include activity sensors, right ventricular impedance for measurement of pre-ejection interval or stroke volume, right ventricular temperature, q-t interval, evoked response gradient potential and other rate responsive methods. Rate responsive pacemakers are often programmed to convert the sensor signal to an appropriate pacing rate modifying signal. Certain parameters of the rate response algorithm, such as the rate slope and maximum pacing rate, are programmable and set up by the physician at implant and during the follow-up.
One limitation of present generation rate responsive pacemakers is that appropriate values for the foregoing parameters are often difficult to determine and, in fact, change over time. One of the goals for a pacemaker is to increase cardiac output by increasing pacing rate. However, as pacing rate increases past a certain point, cardiac output will begin to decrease due to factors such as inadequate ventricular filling. Appropriate pacing rates vary from patient to patient and can change for a given patient over time. The result is that in many cases, rate responsive pacemakers raise the pacing rate to inappropriately high levels. This causes less than optimum cardiac output for the level of activity and physical state of the patient.
Cardiac output and peripheral blood flow sensors are important devices used in various clinical procedures. The most common method for the measurement of cardiac output in the clinical practice today is thermal dilution. Here, a catheter equipped with a thermistor for temperature measurement and an injection port for administration of a saline solution is placed in the heart such that the thermistor is located in the pulmonary artery and the ejection port of the catheter is located in the right atrium. To measure cardiac output a volume of chilled saline is injected into the right atrium. As the chilled saline flows past the thermistor a temperature decrease is detected. The resulting signal is processed to yield blood flow rate information. The technique of thermal dilution has a number of significant disadvantages. Primarily, the greatest disadvantage is that continuous measurement of cardiac output is not possible. Only intermittent values can be determined due to the requirement of chilled saline injection. Secondly, the method is very sensitive to operator technique and this limits the accuracy inherently. A risk of infection is also introduced, given the fact that a foreign liquid is injected into the body.